Noise suppression circuits



July 16, 1946- J. A. WORCESTER, JR 2,404,338

y NOISE SUPPRESSION CIRCUIT v I Filed Aug. 20, 1941 Patented July 16, 1946 UNITED STATES 2,404,3:as v f l Noise sUPPREssIoN olaoUrrs Joseph A. *Worcesten Jr., Bridgeport,z Conn., assignor to General Electric Company, a corporation of New York Application August 20, 1941, Serial No. 407,543

23 Claims. 1

My invention relates to noise suppression circuits and it is particularly applicable to noise suppression circuits for automatically rendering the output circuits of a frequency modulation receiver inoperative when no signals are being received or when the received signals are too weak to provide satisfactory operation.

It is well known that objectionable noises may be emitted by the signal reproducer of a radio receiver during those intervals when no signals are being received, or while the receiver is being tuned between carrier channels. rIhese noises may arise from a great many causes including cross-modulation effects, natural atmospheric static, locally produced high-frequency electrical disturbances, shot" effects and thermal'agitation in the tubes and circuits of the receiver itselfget cetera. Therefore, various squelch circuits have heretofore been proposed for muting the sound reproducer except when the receiver is tuned to a signal of sufficient strength to over-ride the noise level and provide satisfactory reproduction. The operation of such circuits has variously been designated by such terms as background noise suppression, inter-channel noise suppression and carrier-olf noise suppression.

A properly designed frequency modulationv receiver inherently provides a high signal to noise ratio so long as the received signals exceed a predetermined level. This is due in large measure to the action of the amplitude nmiung circuits which transmit the desired frequency modulation but discriminate against amplitude modulation. In order to realize the full possibilities of frequency modulation reception, it is common design practice to provide such a high degree of amplification in the high-frequency stages of the receiver that such effects as thermal agitation in the first tuned circuit and the shot effect in the first tube cause voltages to be impressed on the amplitude limiter approaching values required to saturate it. So long as any signal strong enough to operate the limiter is received, these voltages, as well as other undesired static and noise voltages and undesired amplitude modulation present in the signal, are reduced to a very low level at the limiter output and do not cause objectionable disturbances inthe sound rep-reducer. However, in the absence ofsuch signals it has been found that very substantial noise voltages appear in the limiter output and that, due tothe extremely high sensitivity of the receiver and the frequencycharacteristics of the receiver circuits, the audible background noise may be very severe. A

It is therefore highly desirable to incorporate some form of noise suppression circuits for Veliminating these background" noises in a frequency modulation receiver in order that the full advantages of static-less operation may be real- 5 ized.` Accordingly, it is an important object of my invention to provide an improved background noise suppression circuitwhich is particularly adapted to the requirements of a frequency modulation receiver. 1Q In a preferred embodiment of my invention,

' the noise voltages appearing as amplitude modulationv in the limiter output under no-signal or weak-signal conditions, as explained above, are

detected and utilized to provide bias potentials for 15 reducing the transfer eiliciency of a portion of the transmissionchannel following the limiter. Preferably, one of the'amplifying "stages following the limiter'is completely blocked, and the sound reproducer effectively muted.Y However, as soon as a signalof' sufficient strengthfor satisfactory operation is received (and such signal need be only a relatively weak signal just suflicient to saturate the limiter), the "squelch voltage automatically collapses and the receiver operates at maximum sensitivity.

It is accordingly another'object of my invention'to provide an improved'squelch or muting circuit, particularly adapted to the requirements of a frequency modulation receiver, whose opera- 30, tion is automatically determined by the character and magnitude of the voltages impressed there- It is also a specific object of my invention to provide an improved carrier-off noise suppression circuit for frequency modulation receivers which is very sensitive and has a trigger action.

Still another object of my invention is to provide an improved automatic background noise suppression circuit which is economical and read- 40 ily adapted to existing typesof frequency modulation receivers.

' The features of my invention which I believe to be novel are set forth with particularity in the appended claims. My invention, itself, however, together with further objects and advantages thereof, may best be understood .by reference vto the following description taken in conn'ection with the accompanying drawing, in which Fig. 1 isla schematic representation, partly in the form of a conventionalized one-line diagram, of a frequency modulation receiver embodying my invention; and Fig. 2 schematically represents modified forms of circuitsv embodying my invention which may be substituted for the correspond- `5&5 ing portions of Fig. 1 within thedashed rectangle,

ages, unless otherwise'qualified, to include al1v undesired voltages which may accompanyfthedesired signals or be modulated upon them. `For*l example, these voltages maybe caused bycross" modulation effects, adjacent channel interference or non-linearities in the transmission channel; they may be caused by thermal agitation effects, sho effects, and related phenomena which occur in the receiver itself and give rise to disturbances of the soecalled' continuous or hash typegor they'may beA caused by natural atmospher'icF static, Nignition' interference, andY 'the like,` which' occur outside" the receiver and give rise'to disturbances of the solcalled impulse type. i

Byt'he term amplitude modulation I intend' to 'i'r'iclucle all amplitude variations in the envelope of highl'frequency voltages appearing at specied points 'in the"transmission channel, irrespective of the precise character of these voltages.' Thus,

for -tl'ie'purposes of my'invention these voltages may constitute periodic signal voltages, either unm'odulated or modulated in frequency or phase Y by desired signals, 'or irregular voltages such as are characteristic of any of the types of noise.v

By the'term frequency modulation I mean to include' any form "of modulation produced by Varying the frequency or phase of a carrier wave as 'distinguishedV from' modulation produced by varying the amplitude of such a wave.

VThe generic term angle modulated has been heretofre employed in the art in connection with waves'of this character 'to indicate either'frequency modulatedV or phase modulated waves or waves which 'are hybrids of the two; In the specific example described in the following discussion of the.y drawing, the" received signal energy is Vassumed to befrequen'cy modulated wave energy.'

" 'In' they diagrammatic representation of a frequency modulation receiver Vshown in Fig. 1`, many l.

of the elements maybe conventional and'their details of constr'uction'are not material to my invention'.,` Therefore, to simplify the drawing, such elements 'have'merely been indicated in block form. Thus the receiver illustrated is'for the mostparta conventional superheterodyne receiver. Signals received 'at 'an antenna, Ii] are amplified' in the usual radio frequency amplifiers I I vand combined in a'mixer. I2 with locally generated waves supplied from an oscillator I3. The carrier waves of fixed intermediate frequency are then amplified in intermediate frequency ampli; ners I4. In frequency modulation receiversof usual design it' is often the practice to pass the intermediate frequency waves through two amplitude limiters in cascade in order to secure a better limiting action. Thus in theillustrated' embodimentsomeofj the amplitude modulationis removed in a rst limiter I5 and' practically 'all of the remaining VamplitudeV modulation isf're'- moved in a second limiter I6.V

The intermediateV frequency carrier waves atY the'olltput ofthe secondv limiter I6 'are therefore of substantially constant amplitude'and contain only'theffrequ'ency 'modulation which is repre,- sentative of the intelligence being transmitted,

4 e. g., audio signals. slope filter.l or discriminator I1. Various forms of apparatus suitable for this purpose are well known to the art. One type of frequency modulation detector circuit which I have found'to be satisfactory is described in Patent No. 2,121,103- Seeley,` granted June 21,1938, to WhichV reference maybe `made for further information. L

vThe demodulated audio signals are supplied to ya-rst audio frequency amplifier I8, further amplied in an audio power amplifier I9, and then *I suppliedtoV a suitable signal translating device, h. suoli as the loudspeaker 2U.

current flows and Y .The 'limiter |16 comprises a pentode amplifier 30, havingan input circuit including a coupling capacitor SI and grid resistor 32 connected between its control grid 33 and cathode 34, and an output circuit including a tuned circuit 35 connected to its anode 36. Anode operating potentials are supplied in conventional manner from a suitable source of power, which is not shown but merely indicated by the connection designated as To-PB on the drawing, through the usual decoupling resistor 33. The screen grid 39- is' similarly supplied with operating potentials from the common power supply source through vadecoupling resistor 4U. v v

The operation of the limiter I6 will be readily understood by those skilled in the art without detailed' explanation. The limiter is` self-biased' by means of the capacitor 3I and the grid resistor It is so adjusted as to pass anode current only between the limits at which positive grid' at which anode current cutoff takesplace. 'v v l l In the'form of' my invention illustrated in Fig. 1, the anode 3'6 of the limited Sil-is' alsocoupled to an amplitude detection circuit over a conductor 50.' The circuit comprises a diode detector 5I shunted by a 'load`Y resistor 52'. The intermediate frequency potentials occurring at theanode 36 areimpressed on'the' diode 5I through Ythe conductor 5u anda capacitor A53. This detection circuit functions in a manner shortlyV to be de'- scribed ingreater detail; Brieiiy, it develops rpotentials across the load-resistor 52 in accordance with amplitude modulation components appearing inthe outputof the limiter I6;

Potentials developedacrossv the diode '5IH are impressedy upon a' second. diode detector 54' through acapacitor 55, which removes direct current, and through a'low-pass filter, comprisingY a resistor 56 and a capacitor 5?-, which removes intermediate frequencies.

The load for the diode detector 54 comprises a resistory 5B. Unidirectionalvoltages appearing across this resistor are lfurther filtered by' a lowpass filter; comprising aresistor' 59j and a ca-1 pacitor G6', and impressed between the control grid 1u and cathode'l-I of the first audio ampli@ ii'er tube, which is illustrated as a triode 12; It will be observed that a conductive connection `is provided betweengrid T0 and cathode 'H' through a resistor 6 Iv and resistors 58 and 59'. These'v'oltages are of such polarityas to biasthe `grid 7 0 negatively with respecttothe 'cathodefl"and`, as

will shortly'be:explained; under 'certain conditions their'magnitude is sufiicienttcreduce the `ampli` cation of the audio ampli'er tube 'I2` toa very low level; Preferably tube 'I2A is Vthereby biased completely to cutoff, preventing transmission 'of signals'to the power amplifier I'Sf and therloud speaker20'." f Y i It has beenfoundthat there are always noise voltages present at the second limiter even'when They are demodulated in a no voltages are impressed upon the receiverinput. It has been demonstratedlthat the Ynature of these voltages is that of a spectrum containing an almost infinite number of component frequencies, some of which always coincide with the frequencies associated with the desired signals. It is generally accepted that irregularities in the motion of electrons in the tubes and circuits is largely responsible, i. e., the thermal agitation and shot effects and related phenomena. The first radio frequency amplifier stage undoubtedly contributes the greatest proportion of these disturbances because of the high'gain' provided in the stages between it and the limiter. Furthermore, it has been demonstrated that extraneous disturbances received at the antenna l also comprise an infinite frequency spectrum containing frequencies capable of being translated in the receiver channel.- These disturbances further increase the voltage impressed upon the limiter under no-signal conditions.

As indicated earlier in this specification, it is preferable to provide such a high gain through the radio frequency and intermediate frequency amplifiers that the noise voltages impressed upon the limiter grid 33 develop self-bias potentials approaching values required to saturate the pentode 33 even under no-signal conditions. Nevertheless, under such conditions it has been found that these voltages are not appreciably limited in amplitude in the limiter i6 and that substantial noise voltages are developedat the limiter anode 36 including modulation frequencies-within the audio band.` Reception of external staticf disturbances at the antenna l0 merelyincreases their magnitude. However, as soon as' a carriersignalis received which is suiiicient to maintainrthe limiters continuously saturated, these voltages at the anode 36 almostv completely disappear.' Inl other words,` so long as the received signal is strong enough to over-ride the noise level, voltages within lthe intermediate fre' quency band are continuously impressed upon the limiters to maintain them operative. Y Noise voltages alone, on the other hand, apparently contain no such continuous components, but are more like a highly over-modulated carrier vof practically zero intensity and are not removed by the limiters.

In accordance with my invention these phenomna are utilized to provide background noise suppression in a manner now to be detailed.

In the embodiment of my invention illustrated in' Fig. V1 the vnoisev voltages appearing at the anode 36 are detected inthe diode'l and appear -upon the load resistor 52. Preferably the time constant of this detector circuit is such that the noise voltages appearing on the resistor 52 lie within the audio range, higher frequency components being filtered out. Itis desirable to vuse the audio noise voltages for control since voltages of these frequencies tend to be transmitted through the signal channel following the limiter and to cause audible disturbances in the loud speaker 25. Furthermore, voltages at frequencies within the intermediate frequency pass band are of course not removedby the limiters but are developed at the anode 36 even when signals of satisfactory strength are being received.

Since noise voltages within this range cannot be f y been designated by the same reference numerals and the function of these elements is essentially the same. Therefore, they need not be repeated here. In this modification the pentode 33 is itself utilized to detect the aud-io noise voltages present at the anode 36, permitting the first diode detector to be eliminated. This is possible be'- cause the time constant of the grid circuit' 3|, 32 is such that the pentode 30 functions essentially as a grid power detectorfor amplitude modulation components within the audio frequency band. Therefore, it produces, audio voltages across an audio frequency load in the anode circuit. Such a load is provided by the decoupling network comprising the capacitor 31 and decoupling resistor 38. Consequently, the audio noise voltages appear at the point 13 and maybe supplied directly to the filter and detector-network 5x5-'65, as illustrated. y

In a frequency modulation receiverof` .commercially acceptable sensitivities, the audio noise voltages at ythe point i3 have been found to `be at least of the order-of lO'volts under all operating' conditions. This provides a unidirectional squelch voltage of at least four volts atjthe grid of the audio amplifier 12, which is sufficient Yto cut off a typical high-mu triode driver tube. vExtraneous noise voltages received at the antenna I0 further increase these values so that the squelch voltage is automatically increased when the receiver is operated in excessively'noisy locations.y e

As soon as even a relatively weak signal is received the squelch voltage immediately collapses and the receiver operates at full sensitivity. Such a signal need be only of a few microvolts intensity, i. e.`, just sufficient to render the limiters fully effective. As previously explained, the background noise at the loud speaker is now reduced toa vvery low level through the inherent capabilities of this type of system to discriminate between signals and noise and to reject the latter.

It will therefore be apparent that my invention combines a highly sensitive background noise suppression action with` a fast-acting trigger action. Furthermore, the noise suppression circuit is also `effective in removing side responses caused by the slope characteristics of the sides of the intermediate frequency response curve.4 lIf the receiver is mistuned or misaligned, frequency modulation signals will be detected in the intermediate frequency circuits through Well known slope filter action, producing amplitude modula tion signals across the limiter anode resistor and resulting in the production of a squelch bias in the same manner as noise voltages.

For lcompleteness of illustration only, and not in any sense by way of limitation, the following circuit constants are given as being typical for a frequency modulation receiver of the type represented in Fig. 2. These constants'have been found to be satisfactory yfor the circuit elements of a frequencymodulatio'n broadcast receiver adapted? to receive signals in the vicinity of 42 t'o 5U megacycles'and having' anr intermediate frequency channel tuned to about. 4.3 megacycles. In this particular receiver the .tube 30 was a type 6SJ7 pentode and the audio frequency vamplifier 12, the diode 5I andthe diode 54 were all embodied ina single duo-diode, high-mu triode, type GSQ'T. Other circuit constants follow:

Capacitor 31 mmf `22 Resistor 3'2 e o'hms 180,000 Capacitor 3L` mmf 47 Resistor 38 ohms 22,000 Capacitor 55 r.. mid f .01 Resistor 56 I ohm"s 10,000 Capacitor 57 inmf" 100 Resistor. 58 megohms 1 Resistor 59 do 2.2 Capacitor 6'0 I mf'du .01 Resistor 61 megohms 6.8

V While I have shown particular embodiments of my invention, it will of course, be understood that I do not wish to be limited thereto since various modifications may be made, and I contemplate by the appended claims to. cover any such modi fications as fall Within the true spirit and scope ofl my invention. Y

' What Iv claim as new and desire to secure by Letters Patent of the United States is:

1. In combination, a signal channel for translating signal-bearing waves,.amplitude limiting means in said channel effective substantially to remove amplitude modulation. from waves impressed thereon only so long as a predetermined limiting rlevel is exceeded, and means responsive to amplitude modulation in waves in the output of said limiting means for disabling subsequent portions of said'channel comprising means for rectifying said amplitude modulation to produce a control voltage varying in accordance with the magnitude of said modulation.

2. In combination, a signal translating channel having means therein adapted to translate frequency-modulated high-frequency waves, said means also being effective to supply substantial `z.

noise Vvoltages to its output, amplitude-limiting means coupled to said translatingmeans to limit the amplitude of high frequencywaves supplied through 'said limiting means, said amplitude-limiting means having amplitude modulation components' due y to said noise voltages developedin itsV output when said high-frequency AWaves supplied therethrough are of intensity less than a predetermined level, saidmodulation components being substantially removed from its output so long as the intensity of said waves exceeds said level, means for detecting said rcomponents and deriving a unidirectional potential therefrom, and means responsive to said unidirectional potential for substantially reducing the transfer efliciency mined limiting. level are supplied from said amplifier, output circuits coupledto said limiter, said circuits: including an amplifying device have linga :control electrode, an amplitude. modulation detector also coupled to the'output of rsaid limiter, ;and means for biasing said control'electrode in response to voltages developed by said detector and. in asense to reduce the amplication of said device.

4. In a frequency modulation receiver, the combination comprising a signal amplier, an amplitudeV limiter coupled to said amplifier,` output circuits coupled to. said limiter, said circuits including an electron discharge device having a control electrode', an amplitude modulation detector connected to the output of saidlimiter, and means for biasing said control electrode in response to modulation voltages developed by said detector to prevent translation of saidV signals by said electron discharge device.

5. In combination, a signal amplifier adapted to amplify high frequency voltages modulated in frequency in accordance with a band of signal frequencies, said amplifier having such a high gain that undesired noise voltages of. interfering frequencies are continuously present in its output,` an. amplitude limiter coupled to the output of .said amplifier, said limiter being effective to remove` amplitude modulation, yat signal frequencies, from said 'voltages only so long.r as high frequency voltages exceeding a predetermined limitinglevel are supplied from said amplifier, a signal reproducer coupled to said limiter, and means responsive to vamplitude modulation at signal fre- `quencies in the `output ofv said limiter for preventingoperation of. said reproducer.

6. In radio receiving apparatus for operation on Vhiglfi-frequency Waves modulated in frequency in accordance with a band of signal frequencies, the combination comprising a thermionic ampliner; for said waves having .such av high gain that undesired. noise voltages of interfering frequencies,. due to received disturbances,A thermal agitation,- shot. effects,y and the like, are continuously present in its output, an amplitude limiter coupled to said. amplifier comprising a thermionic device-having an output electrode, said limiter being. of` such' a character that amplitude modulation components. at signal frequencies are continuously present in potentials appearing on said electrode so long as received high-frequency Wavesy do not exceed a predetermined limiting level but are reduced toa negligible'value so long as said waves exceed said level, signal translating circuits including a signal reproducer coupled. to said. electrode, an: impedance element coupled to said electrode and adapted to have voltagesV of signal. frequencies developed thereon in response to. said components, and means for muting said signal; reproducer in response to said last-named voltages.

7. Inradio receiving apparatus for translating high-frequency Waves modulated. in frequency by desired signals, the combination comprising thermionic means for amplifying received waves. to such. a .high level that substantial undesired noise voltages of the frequencies of said signals and due to received. disturbances, thermal agitation, shot effects, and the like, are developed, amplitude limiting means coupled to said amplifying means, said last means being effective to reduce said undesired voltages to a very/low level in its output only so long as high-frequency waves exceeding a predetermined minimum limiting level `areimpressed on its input, means coupled to said outputfor translating said high-frequency waves, andA means responsive to said undesired voltages appearing in said output for disabling said translating means` comprising -means for rectifying 9.5; said voltages to .produce a'control voltagevarylng in accordance .with the lmagnitudeof said undesired voltages." y f 8.'v In a frequency modulation receiver of the type 'comprising -high frequency amplifiers for amplifying waves modulated in frequency by desired signals,an amplitude limiter, a frequency demodulator, Va lowV frequency signal amplifier and a signal reproducer, wherein said high frequency amplifiers provide sufliciently high amplification that undesiednoise voltages -are continuously supplied to said limiter, and wherein said limiter is effective substantially to :prevent rtransmission of noise voltages therethrough only so long as said tially constant level, said means comprising a thermionic device'having an anode circuit, frequency modulation signal detecting means coupled to said anode circuit, an electron discharge amplifier coupled to the output vof vsaid detecting means, said amplifier having a control element, means comprisingV a diode detection circuit for developing audio frequency voltages in response to audio-frequency amplitude modulation components present in Waves developed in said anode circuit, means comprising a second diode detection circuit for developing a unidirectional control potential in response to said audio frequency voltages, and means for biasing said control element negatively in accordance with said control potential.

10. In radio receiving apparatus adapted to translate high-frequency Waves modulated in frequency by desired audio frequency signals, the combination of means for limiting the amplitude of said waves to a substantially constant level, said means comprising a self-biased thermionic limiter having an anode circuit, frequency modulation signal detecting means coupled to said ande circuit and an audio frequency amplifier coupled to the output of said detecting means, said audio frequency amplifier having a grid electrode, a resistance-capacitance network inrsaid anode circuit adapted to have audio frequency noise voltages developed therein, means comprising a diode detection circuit for developing unidirectional control potentials in response to said noise voltages, said circuit having a time constant longer than the period of the lowest desired audio frequency, and means for biasing said grid electrode negatively in accordance with said potentials.

11. In a receiver for carrier Waves frequency modulated by desired signals in which the received carrier waves are first amplified to intensity in excess of a predetermined value and then transmitted through a limiter which limits said waves to a uniform intensity of said value, the method which comprises rectifying noise voltages of the frequencies of said signals and which appear at the output of said limiter during periods of reduced signal intensity, and controlling the yto 10 output from said receiver'in accord with'the rectified'noise voltages.y y

12. In-.a `receiver for carrier waves; frequency modulated by desired signals in which/the' received carrier waves are first amplified to intensityY in excess of a predetermined value and then transmitted through `a llimiter Which-limits said waves to a uniform. intensitylof said value, the

method which comprises controlling theoutput vfrom-said receiver .in accord with noise voltages of varying amplitude and of the lfrequencies of said signals appearing 'atfthe output fof" saidlimiter s during periods of reduced intensity ofithevreceived carrier wave. 1 p L v Y 13.` The combination', in areceiver fofrfrequency modulated carrier/waves of an .amplifier for frequency modulated carrier waves, a .-limiterto limit-waves amplifiedbysaid amplifier to a predetermined value, saidamplifier havingi amplification` s'ufcient to produce noise` voltages of objectionable intensity and variable amplitudeat the output, of said limiter in the absence" of received carrier waves, said noise voltages" disappeeringv from said output in the presenceof re-` ceived carrier Waves of greater thany a predetermined intensity, and means to disable said? Areceiver in response to the amplitude variations of said noise voltages at the' outputjof said` limiter.

14. The combination, in a lreceiver for frequency modulated carrier Waves,l of 'an .amplifier for signal `modulated carrier waves,` a limiter 4to limit waves amplified by said amplier to apr'edetermined value, said amplifier' havingamplilication lsufficient to` produce noise voltages of `objectionable intensity at the output of said limiter in the absence of received carrier Waves, said noise voltages disappearing from said output in the presence of received carrier waves of greater than a predetermined intensity, a rectifier, means to supply said noise voltages to said rectifier, said means including means to eliminate voltages of the frequency of the carrier wave amplified by said amplifier, and means responsive to rectified voltages produced by said rectifier to disable said receiver. v

15. The combination, in a frequency modulation receiver, of an amplifier for frequency modulated carrier waves, a limiter to limit Waves amplified by said amplifier to a uniform intensity, a rectifier connected to rectify noise voltages at the output of said limiter, a second rectifier, means to supply variations in voltage at the output of said first rectifier to said second rectifier and to prevent said carrier waves from reaching said second rectifier, and means responsive to the rectified voltage produced by said second rectifier to reduce the output from said receiver.

16. In a receiver of angle modulated carrier wave energy of the type provided with an amplitude limiter stage, a demodulator and a modulation signal amplifier; the method which comprises detecting from the output of said limiter noise voltage components when the received signal-to-noise ratio is excessively low, rectifying the detected noise voltage components to provide a control voltage, and impairing the operation of said modulation signal amplifier in accordance with said control voltage.

17. A method of receiving frequency modulated waves which comprises limiting the amplitude of the waves, demodulating the limited Waves, amplifying the demodulated Wave energy, detecting from the limiter output noise voltage components in response to the decrease of received wave energy below a usable value, deriving from the de- 18.?In a receiver of angle modulated carrier Vvv'aveienergy `of the type provided with anvamplitudeflimiter stage, the method which comprises detecting from the limiter output noise voltage Vlconiponents when the :received signal-toFnoise ratio is excessively low, rectifying the detected components .to provide a control voltage, and uti.; lizing theco'ntrol Vvoltage to impair receiver reproduction. Y

19. A method of receiving frequency modulated waves which .comprises limiting the amplitude of theA Waves, detecting from the limiter ,output noise voltage components in response to the decrease of received" wave energy below a usable value, deriving `from the noise voltage components .a control voltage, :and preventing reproduction of the wave energy `in response to said control voltage.

.20.' linv a method of frequency modulation reception wherein the amplitude of frequency modulated vcarrier Wavesvis limited in a limiter prior tov detection; :the improvement which comprises deriving a .control voltage from amplitude modu` the 'received :signal to noise ratio is below a pref determined threshold level, and suppressing the reproduction of detected energy in response to 1 .said control 4.voltage thereby to prevent reproduction of noise.

f 'lation appearing in the output of the limiter when 'amplitude limiter, a detector and a modulation amplifier; the improvement which comprises rec,- tifying amplitude modulation noise energy ap. pearing in the limiter output when the angle lmodulated energy decreases 4to -a level such that peakAv carrier voltage is substantially equal to peak noise voltage, deriving a mutingY bias voltagefrom tlie rectii'ied noise energy, and suppressing oper.- ation of the .amplifier with the bias voltagel V22.111 a frequency .modulation receiverof the type having an amplitude modulation limiter, a detectorhand an audio amplier; the improvement which includes means for detecting amplitilde modulation appearing in the limiter output energy in response to adjustment Lof the tuning of said'receiver between stations, means for rec` tifying the noise modulation produced as a result l.of said detection, and further means lfdr muting said gaudio' amplier with the rectified voltage thereby y-to prevent the rush oineisercommonly produced during absence of carrier.

23. A system of receiving angle modulated .carrier energy including means limiting the ,energy to reduce amplitude modulation effects, means demodulatzing the limited energy, vmeans fOr amplifying the demodulated energy, means deriving a :control voltage from amplitude modulation :noise energy appearing inthe output of said limiti-ng means When the received modulated-carrier energy decreases below a `predetermined thresh- Y old, and means for inhibiting said ampliiication with said control voltage.

JOSEPH A. WORCESTER, JR. 

